Paroxysmal Nocturnal Hemoglobinuria (PNH) is a clonal acquired hematologic disorder which results from the incomplete enzymatic assembly of glycosyl- phosphatidylinositol (GPI) anchors used for surface protein attachment. All PNH patients tested have a complementation Class A defect; a cDNA (Piga) that repairs cell lines with the Class A defect has been cloned. Analysis of the Piga gene is therefore important for the understanding of PNH; genetic mutations of abnormal Piga expression could lead to defective assembly of GPI anchors and ultimately to PNH. The overall goal of the proposed research is an investigation of Piga gene mutations and PNH, with emphasis on its abnormal expression in GPI- deficient hematopoietic cells. The first hypothesis is that Piga gene mutations can be identified in all PNH patients. The investigators will characterize Piga expression in PNH patients at the mRNA and protein level, followed by nucleotide sequencing to identify Piga mutations. The investigators will then correlate each Piga gene mutation (genotype) with the clinical expression of GPI-linked surface proteins (phenotype). Three important questions are posed: (1) Do all patients with Type II (partially GPI-deficient) erythrocytes have missense Piga mutations?; (2) Do patients with both Type II and Type III (completely GPI- deficient) erythrocytes have two distinct mutations?; (3) Can cells gain or lose partial expression of GPI- like proteins in vivo? The second hypothesis is that PNH patients have infidelity of DNA replication and overall genetic instability which leads to mutations in the Piga gene. This hypothesis helps to explain why PNH is an acquired disorder, why the Piga mutations are mostly frameshift and may be multiple, and why patients with PNH have an increased risk of leukemia. The third hypothesis is that abnormal Piga expression leads to a cellular growth advantage. The investigators will focus on PNH lymphocytes, and study mechanisms by which GPI-deficient cells gain a growth advantage. The effects of abnormal Piga expression on lymphocyte growth will also be studied by transfection of Piga mutants and inhibition of normal Piga expression.